Add Smooth Chime sound type to buzzer node

[Copilot]

Integrates a two-tone chime sound (D/B notes) into the action category buzzer node as requested.

Changes

• Added "Smooth Chime" sound type to BuzzerNode.SOUND_TYPES
• Implemented smooth chime generator in _generate_buzz_sound():
  • Two pure sine waves: D (587.33 Hz) and B (493.88 Hz)
  • Exponential decay envelope (e^(-3t)) per note
  • 120ms inter-note pause
  • 50ms fade-in, 300ms fade-out
  • Duration normalization via padding/truncation

Implementation

elif sound_type == "Smooth Chime":
    def get_note(freq, note_duration=1.0):
        t_note = np.linspace(0, note_duration, int(fs * note_duration), False)
        note = np.sin(2 * np.pi * freq * t_note)
        envelope = np.exp(-3.0 * np.linspace(0, 1, len(t_note)))
        return note * envelope
    
    ding = get_note(587.33)  # D note
    dong = get_note(493.88)  # B note
    signal = np.concatenate([ding, pause, dong])
    # Apply fade-in/fade-out...

The implementation follows existing sound type patterns and integrates with the node's dropdown selection UI.

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